Direct current motors with four stator poles are known in prior art, especially for driving axial fans. They comprise an outer casing, a rotor with a respective winding, two positively polarized brushes, two negatively polarized brushes and four permanent magnet stators.
Normally in this type of motor, the brushes are identical and spaced at equal angular intervals of 90° about the axis of rotation of the motor and diametrically opposite brushes, with the same polarity, are electrically connected to each other. The permanent magnets, also identical to each other, are made of ferrite and each one is radially aligned with a respective brush. The motor thus has two identical machine sections operating in parallel.
The speed of these motors can be adjusted for example by varying the supply voltage of the brushes using external dissipative elements such as voltage dividers, for example. This type of adjustment, besides being unsatisfactory because of the need to provide the dissipative elements, is also inefficient for all applications where only a limited number of predetermined speeds are required: for example a low speed and a high speed.